The present invention relates to a fast breeder and, more particularly, to a fast breeder which is suitable for reducing the maximum power density in the axial direction of the reactor core.
The fast breeder is a nuclear reactor of the type in which fast neutrons produced as a result of fission in the reactor core are absorbed by a fertile material to produce a new fissile material in the reactor core. Namely, in the fast breeder, the new fissile material is produced at a rate higher than the rate of consumption of the fissile material by fission. It is possible to make an efficient use of the nuclear fuel by this production of the new fissile material, i.e., by the breeding process.
The core of the fast breeder usually has a columnar construction consisting of a driver core region containing the fissile material and a blanket region surrounding the driver core region and consisting mainly of the fertile material. The blanket region has a radial blanket region which surrounds the outer periphery of the driver core region and axial blanket regions which are disposed at both axial ends of the driver core region. The fissile material residing in the driver core region is mainly plutonium 239, while the fertile material in the blanket region consists mainly of uranium 238. This uranium 238 absorbs the fast neutrons produced mainly as a result of the fission of the plutonium 239 and is transformed into plutonium 239.
In the fast breeder, the reactivity is also lost due to consumption of the fissile material after the rated capacity has been achieved. In order to compensate this loss in the reactivity, it has been taken into consideration that the reactivity of the fast breeder at the beginning of burnup is made higher than that during the running operation so that the fast breeder can maintain a predetermined output for a predetermined time period. This reactivity, which has been imparted in advance to the fast breeder so as to maintain the predetermined output, is called "excess reactivity". Control rods to be inserted into the core of the fast breeder control that excess reactivity so as to prevent the nuclear run-away. The control rods of the fast breeder are usually inserted into the reactor core from above.
In order that the energy owned by fuel assemblies loaded in the reactor core may be effectively used, it is necessary to flatten the output distribution as much as possible. In other words, it is required to suppress the maximum power density within a low value.